arXiv subjects

Ewine F. van Dishoeck

Publications and source records attributed to Ewine F. van Dishoeck.

At least 37 records · Page 2Linked to original sources

arXiv2025

MINDS. Anatomy of a water-rich, inclined, brown dwarf disk: lack of abundant hydrocarbons

2MASS J04381486+2611399 (or J0438) is one of the few young brown dwarfs (BD) with a highly inclined ($i\!\sim\!70^\circ$) disk. Here we report results from JWST-MIRI MRS, HST-ACS and ALMA Band 7 observations. Despite its late spectral type (M7.25), the spectrum of J0438 resembles those of inner disks around earlier-type stars (K1-M5, T Tauri stars), with a volatile reservoir lacking hydrocarbons (except for acetylene, C$_2$H$_2$) and dominated by water. Other identified species are H$_2$, CO$_2$, HCN, [Ar$^{+}$], and [Ne$^{+}$]. The dominance of water over hydrocarbons is driven by multiple factors such as disk dynamics, young disk age, low accretion rate and possible inner disk clearing. J0438 appears highly dynamic, showing a seesaw-like variability and extended emission in H$_2 \,\,\, S$(1), $S$(3), $S$(5), [Ne$^{+}$] and CO ($J=3-2$). Interestingly, the CO emission reaches up to 400 au from the brown dwarf, suggesting ongoing infalling/outflowing activity impacting the disk chemistry. These observations underscore the combined power of MIRI, HST and ALMA in characterizing the chemical diversity and dynamics of brown dwarf disks.

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arXiv2025

The chemical diversity of giant-planet nurseries as revealed by ALMA

With the giant exoplanet occurrence rate peaking around stars of 1.5-2 solar masses, there is strong motivation to characterize the disks that set their formation conditions. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to investigate both the availability of different molecules in disks and infer the radial distribution of elemental abundances, enabling us to make connections to exoplanet systems. Here we present a survey of six transition disks around young F-, A-, and B-type stars using ALMA. We find 13C18O, CS, SO, and H2CO in all six systems, as well as ten additional molecules in a subset of disks, including detections of H2S, 33SO, and CH3OCH3. Using these data, and literature data where available, we construct the first comprehensive picture of Herbig disk chemistry. We find clear correlations between molecular tracers of C/O>1 environments (e.g., CS, C2H) and disk mass, as traced by C18O line flux. In contrast, tracers of C/O<1 environments (e.g., SO, CH3OH) do not show significant correlations with disk mass. Interestingly, these molecules are relatively brighter in lower-mass disks, with their presence primarily linked to disks with central cavities and spirals. Finally, we show that the observed chemical diversity seen across Herbig disks leads to varying C/O regimes at the orbital radii of candidate proto-planets identified within these disks. When comparing these inferred disk C/O ratios with those measured for directly imaged exoplanets, we find a notable overlap and show that low C/O gas is common on 10's of au scales in Herbig disks.

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arXiv2025

PROJECT-J: the shocking H2 outflow from HH46

We analyze the H2 emission observed in the HH46 Class I system as part of PROJECT-J (Protostellar Jets Cradle Tested with JWST), to investigate the origin and excitation of the warm molecular outflow. We used NIRSpec and MIRI spectral maps (1.6-27.9 microns) to trace the structure and physical conditions of the outflow. By fitting the H2 rotational diagrams with a multi-temperature gas model, we derived key physical parameters including temperature, extinction, column densities, and the ortho-to-para ratio. This information is combined with a detailed kinematical analysis and comparison with irradiated shock models. We find no evidence of H2 temperature or velocity stratification from the axis to the edge of the outflow, as would be expected in MHD disk-wind models and as observed in other outflows. Instead, the observations suggest that the H2 emission arises from shock interactions between jet bow shocks and/or wide-angle winds with the ambient medium and cavity walls. NIRSpec emission and velocity maps reveal expanding molecular shells, likely driven by the less luminous source in the binary system. We infer an accretion rate of less than 10^-9 solar masses per year for the secondary source, approximately one order of magnitude lower than that of the primary. The H2 emission is consistent with excitation by low-velocity (approximately 10 km/s) J-type shocks, irradiated by an external UV field that may originate from strong dissociative shocks driven by the atomic jet. Future JWST observations will further constrain the evolution of the expanding shell and the mechanisms driving the outflow.

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arXiv2025

MINDS: The molecule-rich disc of the Herbig star HD 35929 revealed with JWST/MIRI

Our knowledge of the chemical composition of the gas in the inner disc of intermediate-mass young stars is limited, due to the lack of suitable instrumentation. The launch of JWST has provided a significant improvement in our ability to probe gas in these inner discs. We analyse the gas composition and emitting conditions of the disc around HD 35929, a young intermediate-mass Herbig star, using MIRI/MRS data. Our goal is to constrain the chemistry and kinematics of the gas phase molecules detected in the inner disc. We use iSLAT to examine the observed molecular lines and DuCKLiNG to detect, fit, and analyse the molecular emission. We find gas phase H2O, CO, CO2, and OH in the disc, as well as HI recombination lines. Surprisingly, we also detect gas phase SiO in the fundamental v=1-0 vibrational band. We derive column densities and temperature ranges of the detected species, arising from the inner ~0.2 au, hinting towards a compact and very warm disc. The molecular column densities are much higher than found in lower mass T Tauri discs. In general, the molecular composition is consistent with an O-rich gas from which silicate-rich solids condense and the strong gas phase molecular line emission suggests a low dust opacity. The unexpected detection of gas phase SiO at the source velocity points to an incomplete condensation of rock forming elements in the disc, suggesting chemical disequilibrium and/or an underestimate of the gas kinetic temperature.

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arXiv2025

MINDS: The very low-mass star and brown dwarf sample II. Probing disk settling, dust properties, and dust-gas interplay with JWST/MIRI

Disks around very low-mass stars (VLMS) provide environments for the formation of Earth-like planets. Mid-infrared observations have revealed that these disks exhibit weak silicate features and strong hydrocarbon emissions. This study characterizes the dust properties and geometrical structures of VLMS and brown dwarf (BD) disks, observed by the James Webb Space Telescope (JWST)/Mid-Infrared Instrument (MIRI), and connects these to gas column density and potential evolutionary stages. We analyze mid-infrared spectra of ten VLMS and BD disks as a part of the MIRI mid-Infrared Disk Survey (MINDS) program. Spectral slopes and silicate band strengths are compared with hydrocarbon emission line ratios, which probe the gas column density. Moreover, the Dust Continuum Kit with Line emission from Gas is used to quantify grain sizes, dust compositions, and crystallinity in the disk surface. The disks are classified into less, more, and fully settled geometries based on their mid-infrared spectral slopes and silicate band strengths. Less-settled disks show a relatively strong silicate band, high spectral slopes, and low crystallinity, and are dominated by 5 $μ$m-sized grains. More-settled disks have weaker silicate band, low spectral slope, enhanced crystallinity, and higher mass fractions of smaller grains. Fully-settled disks exhibit little or no silicate emission and negative spectral slopes. An overall trend of increasing gas column density with decreasing spectral slope suggests that more molecular gas is exposed when the dust opacity decreases due to dust settling. Our findings may reflect possible evolutionary pathways with dust settling and thermal processing or may point to inner-disk clearing or a collisional cascade. These results highlight the need for broader samples to understand the link between dust and gas appearance in regions where Earth-like planets form.

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arXiv2025

Understanding JWST water spectra: what can thermochemical models tell us about the (cold) water in protoplanetary disks?

(Abridged) Rotational H$_2$O spectra as observed with JWST/MIRI provide a good probe of the temperature and column density structure of the inner disk. H$_2$O emission can also be influenced by dynamical processes, such as dust grains drifting inwards and their icy mantles sublimating once they cross the snowlines, thus enriching the inner regions in H$_2$O vapor. Recent work has found that this process may leave an imprint in the H$_2$O spectrum in the form of excess flux in the cold H$_2$O lines. In this work, we aim to test the accuracy of several common retrieval techniques on full 2D thermochemical disk models. Moreover, we investigate the cold H$_2$O emission that has been proposed as a signature of drift, to gain further insights into the underlying radial and vertical distribution of H$_2$O. We present two sets of Dust And LInes (DALI) thermochemical models and run several retrieval techniques to investigate how the retrieved temperature and column density compare to our models. Single-temperature slab retrievals mainly trace the warm ($\sim$500 K) H$_2$O reservoir, whereas a three-component fit is able to better trace the full temperature gradient in the IR emitting region. Retrieved temperatures tend to underestimate the true temperature of the emitting layer due to non-LTE effects. The retrieved column density traces close to the mid-IR dust $τ=1$ surface. We find that the strength of the cold H$_2$O emission is directly linked to the H$_2$O abundance above the snow surface at large radii (>1 au). This implies that sources with excess cold H$_2$O flux likely have a high H$_2$O abundance in this region ($\gtrsim10^{-5}$), higher than predicted by the chemical network. This discrepancy is most likely caused by the absence of dust transport processes in our models, further strengthening the theory that this emission may be a signature of radial drift and vertical mixing.

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arXiv2025

SO2, silicate clouds, but no CH4 detected in a warm Neptune

WASP-107b is a warm (~740 K) transiting planet with a Neptune-like mass of ~30.5 Earth masses and Jupiter-like radius of ~0.94 Jupiter radius, whose extended atmosphere is eroding. Previous observations showed evidence for water vapour and a thick high-altitude condensate layer in WASP-107b's atmosphere. Recently, photochemically produced sulphur dioxide (SO2) was detected in the atmosphere of a hot (~1200 K) Saturn-mass planet from transmission spectroscopy near 4.05 microns, but for temperatures below 1000 K sulphur is predicted to preferably form sulphur allotropes instead of SO2. Here we report the 9-sigma detection of two fundamental vibration bands of SO2, at 7.35 microns and 8.69 microns, in the transmission spectrum of WASP-107b using the Mid-Infrared Instrument (MIRI) of the JWST. This discovery establishes WASP-107b as the second irradiated exoplanet with confirmed photochemistry, extending the temperature range of exoplanets exhibiting detected photochemistry from ~1200 K down to ~740 K. Additionally, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured (~7-sigma) over simpler cloud setups. Furthermore, water is detected (~12-sigma), but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity.

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arXiv2025

MINDS. Strong oxygen depletion in the inner regions of a very low-mass star disk?

JWST is discovering a plethora of species in planet-forming disks around very low-mass stars such as C2H2, C6H6, C4H2, CH3 etc. The column densities of these species retrieved from 0D slab models are very large, e.g. of the order of $10^{20}$\,cm$^{-2}$. This is indicating a carbon-dominated chemistry in a gas with a high C/O ratio. The disk around 2MASS-J1605321-1993159 (M4.5) is one such source showing a molecular pseudo-continuum of C2H2. Still two oxygen-bearing molecules, CO and CO2 are also detected in this source. We aim to take the next step beyond 0D slab models to interpret the spectrum. We examine whether 2D thermo-chemical disk models can produce the large inferred column densities of \ce{C2H2} in the inner regions of the disk and produce a pseudo-continuum in the mid-IR spectrum. We also want to constrain whether depletion of oxygen or enrichment of carbon is causing the high C/O ratio triggering a carbon-dominated chemistry. We utilize the radiative thermo-chemical disk model P{\tiny RO}D{\tiny I}M{\tiny O} to identify a disk structure which is capable of producing the observed molecular emission of species such as CO, CO2, C2H2, and H2O simultaneously. The spectrum is generated using the fast line tracer FLiTs. We derive the gas temperature $\langle T \rangle$, column density $\langle$ log$_{\rm {10}} N\rangle$ and the emitting area $\langle r_{\rm{1}} - r_{\rm{2}} \rangle$ for these molecules from the 2D disk model and compare them to the parameters retrieved originally from 0D slab models. We use the different effect that changing the O or C abundance has on CO and C2H2 respectively to discriminate between O depletion and C enhancement. We find that a disk structure characterised by the presence of a gap can best explain the observations. The inner disk is strongly depleted in dust, especially small grains ($<5\,μ$m), and ..

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arXiv2025

MINDS. Cha Hα 1, a brown dwarf with a hydrocarbon-rich disk

Context. Recent JWST observations have shown that brown dwarfs (BD) are chemically rich, offering valuable insights into giant planet formation. Aims. As part of the MIRI mid-INfrared Disk Survey (MINDS) JWST guaranteed time program, we aim to characterize the gas and dust composition of the disk around the brown dwarf [NC98] Cha HA 1, hereafter Cha H$α$ 1, in the mid-infrared. Methods. We obtain data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 28$μ$m. We use the dust fitting tool DuCK to investigate the dust composition and grain sizes while we identify and fit molecular emission using slab models. Results. Compared with disks around very low mass stars, clear silicate emission features are seen in this BD disk. In addition, JWST reveals a plethora of hydrocarbons, including C$_2$H$_2$, $^{13}$CCH$_2$, CH$_3$, CH$_4$, C$_2$H$_4$, C$_4$H$_2$, C$_3$H$_4$, C$_2$H$_6$, and C$_6$H$_6$ which suggest a disk with a gas C/O > 1. Additionally, we detect CO$_2$, $^{13}$CO$_2$, HCN, H$_2$, and H$_2$O. CO and OH are absent from the spectrum. The dust is dominated by large $\sim$4 $μ$m size amorphous silicates (MgSiO$_3$). We infer a small dust mass fraction ($>$10$\%$) of 5 $μ$m size crystalline forsterite. We do not detect polycyclic aromatic hydrocarbons. Conclusions. Cha H$α$ 1 shows the most diverse chemistry seen to date in a BD protoplanetary disk, consisting of a strong dust feature, 12 carbon-bearing molecules plus H$_2$, and water. The diverse molecular environment offers a unique opportunity to test our understanding of BD disks chemistry and how it affects the possible planets forming in them.

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arXiv2025

A transition from H$_2$O to C$_2$H$_2$ dominated spectra with decreasing stellar luminosity

The chemical composition of the inner regions of disks around young stars will determine the properties of planets forming there. Many disk physical processes drive the chemical evolution, some of which depend on/correlate with the stellar properties. We aim to explore the connection between stellar properties and inner disk chemistry, using mid-infrared spectroscopy. We use JWST-MIRI observations of a large, diverse sample of sources to explore trends between C$_2$H$_2$ and H$_2$O. Additionally, we calculate the average spectrum for the T Tauri ($M_{*}$$>$0.2 $M_{\odot}$) and very low-mass star (VLMS, $M_{*}$$\leq$0.2 $M_{\odot}$) samples and use slab models to determine the properties. We find a significant anti-correlation between the flux ratio of C$_2$H$_2$/H$_2$O and the stellar luminosity. Disks around VLMS have significantly higher $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ flux ratios than their higher-mass counterparts. We also explore trends with the strength of the 10 $μ$m silicate feature, stellar accretion rate, and disk dust mass, all of which show correlations with the flux ratio, which may be related to processes driving the carbon-enrichment in disks around VLMS, but also have degeneracies with system properties. Slab model fits to the average spectra show that the VLMS H$_2$O emission is quite similar in temperature and column density to a warm ($\sim$600 K) H$_2$O component in the T Tauri spectrum, indicating that the high C/O gas phase ratio in these disks is not due to oxygen depletion alone. Instead, the presence of many hydrocarbons, including some with high column densities, points to carbon enhancement in the disks around VLMS. The observed differences in the inner disk chemistry as a function of host properties are likely to be accounted for by differences in the disk temperatures, stellar radiation field, and the evolution of dust grains.

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arXiv2025

MINDS. Young binary systems with JWST/MIRI: Variable water-rich primaries and extended emission

As part of the JWST GTO program MINDS, we analyze the mid-infrared emission of three Class II binary systems: VW Cha, WX Cha, and RW Aur, to investigate the impact of stellar multiplicity on the chemistry and physics of their inner disk. We analyze the 1D spectrum from JWST/MIRI-MRS for primary and secondary disks separately, extracted by combining forward modeling with a theoretical PSF and aperture photometry. We modeled the molecular lines with 0D slab models. We interpret the results by comparing our JWST spectra to VLT/CRIRES+, Spitzer/IRS, and ALMA. Primary and secondary disks are dramatically different in their mid-infrared emission, with primary disks showing H2O-rich spectra, and secondary disks being mostly line poor to the sensitivity of our spectra. When comparing MIRI-MRS to Spitzer/IRS, we observe large variability in the line emission of VW Cha A, as well as in the continuum of RW Aur A. The disks around VW Cha BC and RW Aur B show evidence of ionizing radiation, and a further comparison with ALMA at high angular resolution dust continuum suggest that the spectrum of RW Aur B is well explained by its ~4 au cavity. All the systems show [Ne II] jet emission, and three of them also show spatially resolved emission structures in H2, likely originated by outflows and dynamical interactions. Many of the observed features in the primary disks, such as enhanced water emission, could be linked to the increased accretion and radial drift produced by dynamical disk truncation. However, additional mechanisms are needed to explain the large differences between primary and secondary disks, potentially inner disk substructures. This work is an example of the need for combining multiple facilities to fully understand the observations from JWST.

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arXiv2025

Chemical transformation of CO in evolving protoplanetary discs across stellar masses: a route to C-rich inner regions

Protoplanetary discs around Very Low Mass Stars (VLMS) show hydrocarbon-rich MIR spectra indicative of C/O>1 in their inner discs, in contrast to discs around higher-mass hosts which mostly show O-bearing species. One scenario proposed to elevate C/O in VLMS inner discs is the advection of O-depleted gas from the outer disc. However, if CO gas remains abundant, C/O can be at most ~1. We test if chemical transformation of CO into other species allows this transport scenario to produce C/O significantly above 1. We track the evolving inner disc C/H and O/H with a 1D disc evolution code. We model the transport of molecules in gas and ice and add conversions of species to represent key reaction pathways at the midplane. We explore the role of disc mass, size, ionization rate, and substructures. The inner disc C/O increases over time due to sequential delivery where O-rich species (e.g. H2O) give way to C-rich species (e.g. CH4). To reach C/O>1, separating C and O is key, hence the liberation of C from gaseous CO by He+ is critical. Ionization drives this chemistry and needs rates >~10^-17 s^-1 for VLMSs for sufficient chemical evolution within a disc lifetime. However, <~10^-17 s^-1 is needed to ensure that C/O stays <1 for the first few Myr in T Tauri discs. While C/O is usually higher for VLMS than T Tauri stars due to faster sequential delivery, C/O significantly above 1 is only produced by combining gas-phase CO destruction with gas advection and radial drift. Sufficient O depletion and hydrocarbon production around VLMSs can then be achieved but may imply higher ionization rates than T Tauris. Observations of older discs may distinguish whether higher ionization rates are indeed needed or faster physical evolution timescales alone are sufficient. CH3OH ice photodissociation at a dust trap between the CH3OH and CH4 snowlines may also liberate C as CH4 gas that can enrich the inner disc.

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arXiv2025

HDO ice detected toward an isolated low-mass protostar with JWST

Water is detected in environments representing every stage of star and solar system formation, but its chemical evolution throughout these stages remains poorly constrained. Deuterium ratios offer a means of probing chemical links between water in different cosmic regions because of their sensitivity to physicochemical conditions. Here, we present the first detection of the 4.1 $μ$m HDO ice feature with JWST toward a low-mass protostar, L1527 IRS, which may eventually grow to a sun-like mass. We measure an ice HDO/H$_{2}$O ratio of 4.4$^{+3.7}_{-1.7}$$\times$10$^{-3}$, where the reported error is dominated by uncertainties in continuum definition and ice band strengths. This fraction is similar to the gas HDO/H$_{2}$O ratios measured in the warm ($>$100 K) inner cores of other low-mass protostellar envelopes and protoplanetary disks found in comparably isolated star-forming regions. Such a similarity tentatively supports the assumption that water vapor detected in these regions is not significantly altered by gas-phase reactions following ice sublimation. It also supports the hypothesis that pre- and protostellar water ice is largely inherited in a chemically unaltered state by outer protoplanetary disks. However, the fraction is a factor of $\sim$4-10 times higher than the gas HDO/H$_{2}$O ratios measured toward comets and low-mass protostars in clustered star-forming regions. This difference may be due to either gas-phase water reprocessing in protostellar envelopes and protoplanetary disks, or differences between prestellar conditions of isolated dense cores and the clustered star-forming regions that are more analogous to the environment in which our Sun formed.

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arXiv2025

MINDS. Water reservoirs of compact planet-forming dust disk: A diversity of H$_2$O distributions

Millimetre-compact dust disks are thought to have efficient radial drift of icy dust pebbles, which has been hypothesised to produce an enhanced cold ($T<$400 K) H$_2$O reservoir in their inner disks. Mid-infrared spectral surveys, now with the James Webb Space Telescope (JWST), pave the way to explore this hypothesis. In this work, we test this theory for 8 compact disks ($R_\mathrm{dust}<$60 au) with JWST-MIRI/MRS observations. We analyse the different reservoirs that can be probed with the pure rotational lines ($>$10 $\mathrmμ$m) through parametric column density profiles, multiple component slab models, and line flux ratios. We find that not all compact disks show strong enhancements of the cold H$_2$O reservoir, instead we propose three different classes of inner disk H$_2$O distributions. Four of our disks appear to have similar H$_2$O distributions as many of the large and structured disks (Type N or ``Normal'' disks), as is indicated by the slab model fitting and the line flux ratios. These disks have a small cold reservoir, suggesting the inward drift of dust, but it is not as efficient as hypothesised before. Only two disks do show a strong enhancement of the cold H$_2$O emission (Type E or cold H$_2$O enhanced disks), agreeing with the original hypothesis. The two remaining disks are found to be very H$_2$O-poor (Type P or H$_2$O-poor disks), yet show emission from either the hot or immediate reservoirs (depending on the fit) in addition to emission from the cold one. We find that different parametrisations are able to provide a good description of the observed H$_2$O spectra, with the multiple component analysis yielding similar results. Finally, we also report the detection of other molecules in these disks, including a tentative detection of CH$_4$ in CY Tau.

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arXiv2025

MINDS: The very low-mass star and brown dwarf sample -- Hidden water in carbon-dominated protoplanetary disks

Infrared observations of the inner disks around very low-mass stars (VLMS, $<$0.3$\,M_{\odot}$) have revealed a carbon-rich gas composition in the terrestrial planet-forming regions. Contrary to the typically water-rich T Tauri disk spectra, only two disks around VLMS have been observed to be water-rich among more than ten VLMS disks observed so far with JWST/MIRI. In this letter, we systematically search for the presence of water and other oxygen-bearing molecules in the JWST/MIRI spectra of ten VLMS disks from the MIRI mid-INfrared Disk Survey (MINDS). In addition to the two previously reported detections of water emission in this VLMS sample, we detect water emission in the spectra of three other sources and tentatively in one source, and we provide strong evidence for water emission in the remaining disks in the MINDS sample, most of which have bright emission from carbon-bearing molecules. We show that the $\rm C_2H_2$ emission is much stronger than that of water for sources with low luminosities, and the hydrocarbons outshine the water emission in such conditions. We propose that the appearance of water-rich vs. hydrocarbon-rich spectra is related to the location of the water reservoir in the disk relative to the main hydrocarbon reservoir. Our findings indicate that the terrestrial planet forming regions in VLMS disks have high carbon-to-oxygen ratios (C/O$>$1), but can still harbor ample water similar to those in the T Tauri disks.

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arXiv2025

Unveiling two deeply embedded young protostars in the S68N Class 0 protostellar core with JWST/NIRSpec

The near-infrared (NIR) emission of the youngest protostars still needs to be characterized to better understand the evolution of their accretion and ejection activity. We analyze James Webb Space Telescope NIRSpec 1.7 -- 5.3 $μ$m observations of two deeply embedded sources in the S68N protostellar core in Serpens. The North Central (NC) source exhibits a highly obscured spectrum (A_K ~ 4.8 mag) that is modeled with a pre-main-sequence photosphere and a hot disk component. The photospheric parameters are consistent with a young, low-mass photosphere, as suggested by the low surface gravity, log g of 1.95 $\pm$ 0.15 cm s$^{-2}$. The hot disk suggests that accretion onto the central protostellar embryo is ongoing, although prototypical accretion-tracing emission lines HI are not detected. The South Central (SC) source, which is even more embedded (A_K ~ 8 mag; no continuum is detected shortward of 3.6 $μ$m) appears to be driving the large-scale S68N protostellar outflow, and launches a collimated hot molecular jet detected in \Ht and CO ro-vibrational lines. Shock modeling of the \Ht (ro)vibrational lines establishes that fast $C$-type shocks ($\geq$ 30 km s$^{-1}$), with high pre-shock density ($\geq$ $10^7$ cm$^{-3}$), and strong magnetic field (b ~ 3--10, where $B = b\,\times\,\sqrt{\textrm{n}_{\textrm{H}} (\textrm{cm}^{-3})}\,μ\textrm{G}$) best match the data. The bright CO fundamental line forest suggests energetic excitation, with the contribution of non-LTE effects, ie irradiation pumping. Detected OH and CH$^{+}$ ro-vibrational lines support this hypothesis. These two Class 0 protostars seem to be in very young evolutionary stages and still have to acquire the bulk of their final stellar masses. These results demonstrate that JWST enables unprecedented diagnostics of these first stages of the protostellar evolutionary phase.

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arXiv2025

HCN and C2H2 in the atmosphere of a T8.5+T9 brown dwarf binary

T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H2O, CH4 and NH3, which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn informs our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum (R ~ 1500-3000) of a T-dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P-T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H2O, CH4 and NH3 features. Moreover, we make the first detections of HCN and C2H2 (at 13.4$σ$ and 9.5$σ$ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing ($K_{zz}\sim10^{11}$cm$^2$s$^{-1}$), challenging previous literature derivations of $K_{zz}$ values for T-type brown dwarfs. Even more surprising is the C2H2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing, and/or our understanding of the C2H2 chemical network, or might hint towards a more complex atmospheric processes such as magnetic fields driving aurorae, or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

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arXiv2025

First unambiguous detection of ammonia in the atmosphere of a planetary mass companion with JWST/MIRI coronagraphs

The newly accessible mid-infrared (MIR) window offered by the James Webb Space Telescope (JWST) for exoplanet imaging is expected to provide valuable information to characterize their atmospheres. In particular, coronagraphs on board the JWST Mid-InfraRed instrument (MIRI) are capable of imaging the coldest directly imaged giant planets at the wavelengths where they emit most of their flux. The MIRI coronagraphs have been specially designed to detect the NH3 absorption around 10.5 microns, which has been predicted by atmospheric models. We aim to assess the presence of NH3 while refining the atmospheric parameters of one of the coldest companions detected by directly imaging GJ 504 b. Its mass is still a matter of debate and depending on the host star age estimate, the companion could either be placed in the brown dwarf regime or in the young Jovian planet regime. We present an analysis of MIRI coronagraphic observations of the GJ 504 system. We took advantage of previous observations of reference stars to build a library of images and to perform a more efficient subtraction of the stellar diffraction pattern. We detected the presence of NH3 at 12.5 sigma in the atmosphere, in line with atmospheric model expectations for a planetary-mass object and observed in brown dwarfs within a similar temperature range. The best-fit model with Exo-REM provides updated values of its atmospheric parameters, yielding a temperature of Teff = 512 K and radius of R = 1.08 RJup. These observations demonstrate the capability of MIRI coronagraphs to detect NH3 and to provide the first MIR observations of one of the coldest directly imaged companions. Overall, NH3 is a key molecule for characterizing the atmospheres of cold planets, offering valuable insights into their surface gravity. These observations provide valuable information for spectroscopic observations planned with JWST.

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